1. Field of the invention
The present invention relates generally to a variable gain amplifiers, and more particularly, to a variable gain amplifier capable of ensuring a wide gain-controllable range.
2. Description of the prior art
A circuit diagram showing an RDS signal decoder IC is described in an article "Development of Radio Data System Decoder IC's" in page 383-394 of IEEE Transactions on CONSUMER ELECTRONICS, Vol. CE-33, No. 3 issued on Aug. 26, 1987. A variable gain amplifier for making the level of an RDS signal constant is arranged in the RDS signal decoder IC. The above described variable gain amplifier has such a structure as shown in FIG. 3.
In FIG. 3, an input signal Vi to be subjected to gain-control is applied to an input terminal 1 and is then amplified in a first stage amplifying portion 4 including first and second transistors 2 and 3. At that time, a gain-controlling signal Vc is applied to a control input terminal 5 of the first stage amplifying portion 4. Accordingly, a gain of the first stage amplifying portion 4 becomes to be corresponding to the gain-controlling signal Vc. An output signal V.sub.01 of the first stage amplifying portion 4 is taken-out from collectors of the first and second transistors 2 and 3 which are double- ended, being applied to a subsequent stage amplifying portion 6. The output signal V.sub.01 is further amplified in the subsequent stage portion 6 and then, provided to an output terminal 7.
On the assumption that R.sub.E is a resistance value of a resistor 8 connected between emitters of the first and second transistors 2 and 3 and I.sub.l is a collector current of each of constant-current transistors 9 and 10, the gain G.sub.l of the first stage amplifying portion 4 is as follows: ##EQU1## where K is the Boltzmann constant, T is the absolute temperature, and q is the charge on electrons.
Furthermore, let 2I.sub.2 be a current flowing through a constant-current source 11 in the subsequent stage amplifying portion 6 and let R.sub.L be a resistance value of a load resistor 12, a gain G.sub.2 of the subsequent stage amplifying portion 6 is as follows: ##EQU2## where V.sub.O is an output voltage obtained at the output terminal 7.
From the above described equations 1 and 2, a total gain G of the circuit shown in FIG. 3 is as follows: ##EQU3## Here R.sub.E &gt;&gt;2KT/qI.sub.l because 2KT/qI.sub.l is an internal resistance of the emitter of the transistor. Accordingly, the total gain G is approximately proportional to I.sub.2 /I.sub.l. Consequently, if the control signal Vc applied to the control input terminal 5 is varied, the current Il is changed in response thereto , so that the total gain G is changed. Therefore, the circuit shown in FIG. 3 can be used as a variable gain amplifier.
As obvious from the above described equations 3, a maximum gain of the variable gain amplifier shown in FIG. 3 is determined by the resistance value R.sub.E of the resistor 8. The smaller the above resistance value R.sub.E is, the larger the maximum gain can be.
In addition, an input dynamic range of the above described variable gain amplifier shown in FIG. 3 is determined by a product of the resistance value R.sub.E of the resistor 8 and the current I.sub.l flowing through the constant-current transistors 9 and 10, i.e., R.sub.E.I.sub.l. The larger the above resistance value RE is, the larger the input dynamic range can be.
Furthermore, a gain-controllable range of the variable gain amplifier shown in FIG. 3 is determined by an inherent maximum current value being dependent on resistance components of diodes 13 and 14, saturation of the first and second transistors 2 and 3 and etc.
Therefore, the resistor 8 is associated with all characteristics such as a maximum gain, an input dynamic range, and an gain-controllable range. On the other hand, a direction of a change in value of the resistor 8 and a direction of improvements in the above characteristics do not necessarily coincide with each other. Accordingly, it is very difficult to select a resistance value at which the above characteristics are all the best.